Method of fabricating a tube sheet assembly

ABSTRACT

A tube sheet assembly has tube sheets with a plurality of orifices formed therein with an enlarged bore inner portion extending partially through the thickness of the sheet for slidably receiving the tube ends intersecting with a coaxial smaller outer bore portion extending through the rest of the sheet thickness corresponding to the inner diameter of the tube. With the ends of each tube positioned in the enlarged inner bore portion of the respective orifices, an explosive welding charge is inserted through the smaller outer bore portion to be positioned within the end of each tube. The charges may then be detonated simultaneously to weld the outer surfaces of the tube ends to the surrounding inner tube sheet surfaces within the enlarged bore portion of the orifices, thus providing an essentially uniform fluid flow path free of discontinuities from the tube sheet into the tubes.

United States Patent 51 June 27, 1972 Lieberman [54] METHOD OFFABRICATING A TUBE SHEET ASSEMBLY [72] Inventor: Irving Lieberman,Covina, Calif.

[73] Assignee: Whittaker Corporation, Los Angeles,

Calif.

[22] Filed: March 20, 1970 [21] Appl.No.: 21,435

[52] U.S.Cl. ..29/47l.3,29/42l,29/479, 29/486 [51] Int. Cl ..B23k 31/02[58] Field ofSearch ..29/47l.1,497.5, 157.4, 471.3,

[56] References Cited UNITED STATES PATENTS 2,962,805 12/1960 Heimberger..29/157.4

3,409,969 11/1968 Simons et al....

3,503,110 3/1970 Berry et al.

3,535,767 10/1970 Doherty, Jr etal 3,551,995 l/l97l Marechal ..29/47l 1Xllgl Primary Examiner-John F. Campbell Assistant Examiner-RichardBernard Lazarus Attorney-Donald E. Nist [57] ABSTRACT A tube sheetassembly has tube sheets with a plurality of orifices formed thereinwith an enlarged bore inner portion extending partially through thethickness of the sheet for slidably receiving the tube ends intersectingwith a coaxial smaller outer bore portion extending through the rest ofthe sheet thickness corresponding to the inner diameter of the tube.With the ends of each tube positioned in the enlarged inner bore portionof the respective orifices, an explosive welding charge is insertedthrough the smaller outer bore portion to be positioned within the endof each tube. The charges may then be detonated simultaneously to weldthe outer surfaces of the tube ends to the surrounding inner tube sheetsurfaces within the enlarged bore portion of the orifices, thusproviding an essentially uniform fluid flow path free of discontinuitiesfrom the tube sheet into the tubes.

5 Claims, 3 Drawing Figures l/197l Brown eta] ..29/497.s x

METHOD OF FABRICATING A TUBE SHEET ASSEMBLY BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to animproved method of forming tube sheet assemblies, and more particularly,to such a method employing explosive welding techniques for joining thetube ends to the tube sheets.

2. Description of the Prior Art Many types of conventional heatexchangers, such as boilers, condensors, coolers and the like, employtube sheet assemblies to transfer heat between two fluids flowing onopposite sides of the tube walls. While one fluid flows through theinterior of the tubes, the other fluid at a different temperature flowsthrough the heat exchange space past the exterior of the tubes boundedby the inner tube sheet surfaces. Heat is transferred due to thetemperature difference through the tube walls formed of a suitable metalhaving good heat conductivity. For any given set of heat transferparameters, including temperature difference between the fluids,conductivity of the tube material, tube dimensions, and fluid flowrates, the overall heat exchange efi'rciency improves with the number oftubes employed to carry one of the fluids. Obviously, the larger thenumber of such tubes, the greater is the flow capacity and the greaterthe area of the heat exchanging surfaces between the fluids, thusproviding a higher rate of heat transfer.

In many heat exchangers, the tubes are affixed to extend inwardly acrossa heat exchange space defined between two opposing tube sheets, to whichthey are attached at their opposite ends. In such assemblies, orificesformed in both tube sheets at the tube ends provide direct flowcommunication between the tube interiors and the headers or fluid filledenclosures covering the outer tube sheet surfaces, and the tube endsmust be welded or otherwise sealed to the tube sheets at each orificesto provide a fluid tight flow path between tube sheets. In other typesof heat exchangers, U-shaped tubes have their ends inserted in orificesformed in separate inlet and outlet manifold areas defined adjacent theouter tube sheet surface.

Conventional techniques used heretofore for sealing numerous tubes totheir tube sheets have been difiicult and time consuming. Any flow pathdiscontinuities at the tube ends, particularly the inlet, can result inundesirable turbulence and non-laminar flow to erode tube walls andsurrounding surfaces and reduce the flow rate and overall heat exchangeefficiency.

. Previously the tubes have been attached to the sheets by either of thetwo basic methods. In one the diameter of the tube sheet orifices areslightly greater than the outer diameter of the tube ends which areinserted into the orifices from the inner tube sheet surface. With theannular surfaces of the tube ends held flush with the outer surface ofthe tube sheet, each tube must then be individually welded around itsentire perimeter to effect the required fluid tight seal with the tubesheets. The usual method of welding the tube ends to outer tube sheetsurface causes flow path discontinuities at the tube ends due to theirregular weld surfaces, resulting in erosion of the tube walls and theweld. A fillet weld applied around the inwardly projecting tubes on theinner tube sheet surface avoids the irregular weld contours in the flowpath at the tube entrances, and if the tube ends are carefully machined,cut to exact size and carefully positioned in the tube sheet, a smoothflat surface surrounding the tube entrances can be provided. However, intube sheet assemblies having numerous closely spaced tubes it may beimpossible or extremely difiicult to weld the interior tubes on theinner tube sheet surface by reaching between surrounding tube alreadyinstalled, particularly since tubes cannot be inserted one at a time forwelding in opposing tube sheets. Accordingly, the weld must usually beapplied to the tube ends at the outer tube sheet surfaces where theirregularity of the weld causes turbulence that gradually erodes both itand the tubes.

An alternative technique sometimes used involves providing tube sheetshaving orifices with the same diameter as the tube interior. The tubeends must then be exactly positioned in registration with the orificeagainst the inner surface of the tube sheet while the weld is appliedaround it. Although this allows each tube to be separately positionedand welded to opposing tube sheets at the same time, thus avoiding thedifiiculty of reaching interior tubes when they are already surroundedby others, a much weaker structure results because the individual tubesare supported solely by the weld at the tube sheet surface and not atall by the surrounding tube sheet. The entire strength of the structuredepends on the inherently weak fillet welds joining the tube ends to thesheet.

SUMMARY OF THE INVENTION Orifices formed through the tube sheets have anenlarged counterbore inner portion with a diameter slightly larger thanthe outer diameter of the tube end to be placed therein and a smallerouter bore portion with a reduced diameter cor responding to the innerdiameter of the tube. The opposite ends of each tube are inserted intothe counterbore portion of the respective orifices, and a suitableexplosive welding charge is inserted from the outer surface through thetube sheet into the end of each tube. The charges inserted may bedetonated substantially simultaneously to form a metallurgical bondbetween the outer tube end surfaces and the abutting tube sheet surfaceswithin the counterbore portions of each orifice. This assembly hasincreased structural strength as a result of the large weld areas withsmooth flow paths at the tube entrances to prevent turbulence andnon-laminar flow from eroding the interior tube walls and from reducingflow rates and heat exchange efliciency.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of asimple tube sheet assembly for a heat exchanger such as may beconstructed in accordance with the invention;

FIG. 2 is a cross-sectional view illustrating a portion of a tube sheetwith a tube end inserted and the explosive welding charge positionedprior to detonation in accordance with the invention; and

FIG. 3 is a view of a completed assembly portion of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1, whichillustrates a simple tube sheet assembly such as might be employed in atypical liquid heat exchanger, a pair of flat tube sheets 10 arepositioned in opposing spaced apart parallel relationship. Both tubesheets 10 have identical mirror image patterns of orifices 12 extendingthrough the thickness of each tube sheet between an inner surface 14 andan outer surface 15. The space between the inner tube sheet surfaces 14in the complete heat exchange assembly constitutes a heat exchangeenclosure to be filled with one of the fluids. The outer surfaces 15 ofthe tube sheets 10 each define one wall for the fluid filled enclosures,sometimes referred to as headers in certain heat exchange applications,that conduct the other fluid into and out of a number of heat exchangertubes 18 that extend through the heat exchange enclosure. As is wellknown, for certain multiple pas heat exchangers, the header or outerenclosures would be partitioned into separate spaces each surrounding adifferent group of holes 12 arranged together on the tube sheet.

As shown in FIG. 2, each of the holes or orifices 12 are drilled orotherwise formed with an enlarged counterbore portion extendingpartially through the thickness of the tube sheet 10 from its innersurface 14 so that the enlarged bore diameter is capable of slidablyreceiving the end of one of the tubes 18. Heat is exchanged between theone fluid filling the heat exchange enclosure and the other fluidflowing in the pipes 18 by conduction through the metal tube walls. Asmaller bore portion 22 of the orifice 12 extends through the remainingthickness of the tube sheet 10 to the outer tube sheet surface 14.

The enlarged counterbore section 20 of the holes 12 preferably has adiameter slightly larger than the outer diameter of the end of the tube18 which is slidably received therein, and it extends approximatelyhalfway through the thickness of the tube sheet to form an annularshoulder 24 intermediate the inner and outer tube sheet surfaces 14 and15. The smaller bore section 22 should have a diameter approximatelyequal to or slightly smaller than the inner diameter of the tube 18. Asshown in FIG. 2, the end of each tube 18 is inserted into thecounterbore section 20 of its respective hole 12 at the inner tube sheetsurface until its annular end surface abuts against the annular shoulder24.

In fabricating this type of tube sheet assembly, as shown in FIG. 1, themetal tubes 18 may be cut to standard length from an availabe longerpipe stock. Since the tube ends in the finished assembly will beshielded from the flow stream by the shoulder 24, as shown in FIG. 3,the cut end surfaces need not be machined to a smooth surface. Theorifices 12 may be drilled or otherwise formed in the tube sheet 10. Oneend of each tube 18 is inserted into a corresponding orifice 12 in afirst one of the tube sheets 10 from its inner surface 15. With all ofthe tubes 18 inserted to extend outward from the first tube sheet 10,the other tube sheet 10 may then be positioned with its orifices 12 inregistration with the opposite free end of each tube. With the free endsof all tubes 18 axially aligned with their respective orifices 12 at theinner surface 15, the other tube sheet 10 can be pushed inwardly toslidably engage the free ends of the tubes 18 for insertion into therespective counterbore sections 20. Alternatively the tube ends mayalready be welded in place in the first tube sheet before inserting thefree ends into the other. However, with the tube ends inserted into bothtube sheets 10, the tube sheets may be clamped or otherwise held inposition against the opposite ends of the tube 18. With a single tubesheet assembly employing U-shaped tubes, the opposite tube ends aremerely inserted into the appropriate orifices on the inlet and outletportrons.

As shown in FIG. 2, with the tube ends in place, an explosive weldingcharge 50 is inserted from the outer tube sheet surface 14 through thesmaller bore portion 22 of the respective orifice 12 into the end ofeach tube 18. In this case, the welding charge 50 has an annular shapedmain charge 55 that fits into the tube end with a disc shaped detonatingcharge 56 bonded at one end to cover the cylindrical opening through themain charge 55. An electrically actuated detonator cap 58 is affixed tothe center of the disc shaped detonator charge 56 with lead wires 60connected to carry on electrical current pulse from an apprOpriate powersource 62. High speed buming explosive, such as Primacord, can be usedto interconnect adjacent charges to avoid the expense of numerousdetonator caps 58.

The type of explosive employed to perform the welding operation and thedimensions of the main charge 55 depend primarily on the type of metalsemployed in the tube 18 and tube sheets 10, the thickness and diameterof the two walls, and the depth of the enlarged counter bore portion aswell as the ratio which exists between the inner diameter and the tubewall thickness. For any particular tube assembly operation, suitableexplosive welding charge configurations may be selected in accordancewith the conventional explosive welding techniques used in joining acylinder within a cylinder, and the most effective configuration can beascertained from individual experimental trials. The annular shaped maincharge 55 might be a hollow cylinder of pressed or cast explosive, or athin walled plastic cylinder containing a powdered explosive.Preferably, as shown in FIG. 2, the longitudinal dimension of explosivewelding charge 50 generally should be less than the depth of theenlarged counterbore 20 so that the charge 50 can be placed within theend of the tube 18 with the inner end of the annular main charge 55slightly recessed within the tube sheet 10 from the inner surface 15.This prevents the full explosive force from being applied to the tubewalls outside of and thus not supported by the surrounding tube sheet10, which might otherwise be puffed outward and possibly even ruptured.When an actuating current pulse from the source 62 is applied to thedetonator cap 58, or the detonation shock from the Primacord", the discinitiation charge 56 bums rapidly outward from its center at asubstantially uniform rate in all directions to ignite the bonded outerend surface of the annular main charge 55 about its entire circumferenceat approximately the same time. The main charge 55 then burnslongitudinally propagating a shock wave along the tube inwardly from itsend. This shock wave forces the surrounding tube walls radially outwardagainst the inner surface of the enlarged counter bore portion of thetube sheet orifice 12 with the shock generated jet from the outersurface of the tube effecting the metallurgical bonding between theabutting metal surfaces of the outer tube wall and inner wall of tubesheet hole. To achieve the jetting effect, the outer tube walls shouldbe spaced from the inner bore surfaces, as by use of a separator tapearound the tube end or, as illustrated, a spacer ridge 57 within thebore.

In a tube sheet assembly, such as shown in FIG. 1, containing a numberof closely spaced tubes, the explosive welding charge 50 for all or asubstantial number of the tubes can be inserted to be detonatedsimultaneously welding all of the tube ends to the tube sheet 10 in asingle operation, as compared with the time consuming and tediousindividual welding of each tube required heretofore. Shock wavesradiated outward from surrounding tube positions would be transmittedthrough the tube sheet 10 forcing the surrounding orifice walls inwardagainst the respective ends to further assist the welding operation.However, in larger tube sheet assemblies containing great numbers oftubes, the number of tubes welded in a single simultaneous operation mayhave to be limited if the total blast force from the tube sheet mightcause damage or injury in the vicinity, particularly when such weldingoperations are to be conducted indoors.

As shown in FIG. 3, at the detonation of the explosive welding charge50, the tube walls within the tube sheet orifice 12 are forced outwardlyagainst the inner walls of the enlarged counter bore section 20 fillingthe previously surrounding spaces. The cut end surfaces 24 of the tubes18 are expanded outwardly to be shielded by the shoulder 42. Thisremoves any rough tube end surfaces from the flow path between the tubesheet 10 and tube 18 to keep it virtually free of discontinuities thatmight otherwise cause turbulence and non-laminar flow at the tubeentrances to erode the interior tube surfaces.

Moreover, as compared with the prior use of single fillet welds betweeneach tube and the inner surface of the tube sheet, the weld provided bythe method of this invention is much stronger since a greater weld areabetween the tube sheet and the tube end exists and thus provides anoverall improvement in the structural strength of the tube sheetassembly. Also, the tedious task of individually welding each tube intothe tube sheet is avoided, and numerous tubes can be welded in a closelyspaced pattern without irregular contours in the flow path or having tomanipulate welding equipment to reach points on the inner surfacesurrounding interior tubes. Of course, the advantages of the inventionare equally applicable with minor modification to diverse types of tubesheet assemblies and the like which, for example, might include thewelding of boiler tubes into the cylindrical surface of a header,instead of a flat tube sheet as shown in FIG. 1

In one particular tube sheet assembly constructed in accordance with theinvention for use in a high pressure boiler preheater, a carbon steeltube sheet had about 2,000 orifices drilled through an approximatelyS-inch sheet thickness. The enlarged counterbore portion had a diameterof 0.630 inch to a depth of approximately 1 inch. U-shaped steel tubes18 each having an internal diameter of 0.445 inch and an externaldiameter of 0.625 inch were each cut to a length of approximately 10 ft.The explosive welding charge 50 consisted of an annular shaped maincharge 55, approximately 0.750 inches long cut from a :-inch thick sheetof conventional PETN impregnated flexible plastic and rolled into acylinder. A circular ignitor disc cut from the same /fi-inch sheet wasbound by adhesive to one end of the rolled cylinder with the initiatingexplosive cord strand in lieu of the detonator cap affixed by adhesiveat its center. The explosive welding charge so formed was positioned ineach tube with the inner surface of the annusmallerouter bore portion isformed to correspond to the inner diameter of the end of the tubeinserted therein, and the diameter of said enlarged counterbore portionis formed to be slightly larger than the outer diameter of the end ofthe tube lar main charge 55 recessed approximately 0.125 inches from 5inserted therein.

the tube sheet inner surface 15. Because the work was performed in anindoor factory facility, only two hundred of the explosive charges wereignited at one time to limit the blast effect within the confined space.To avoid the need for a separate initiator cap 58 for each charge, onlya single cap was used with the remainder of the 200 charges being linkedto it by high speed explosive cord extending between and affixed to thecenter of the discs 56. Thus, the approximately two thousand tube endscould be welded to the tube sheet in several operations each involving amaximum of 200 tubes at a time. The resulting welds between each tubeand the tube sheet were found to have greater strength than conventionalfillet welds and provided a flow path at the tube ends having asubstantially uniform surface without detectable surface discontinuitiesto cause turbulence or non-laminar flow.

What is claimed is: l. A method of forming a tube sheet assembly havinga plurality of tubes extending therefrom comprising:

forming a tube sheet with a plurality of orifices extending therethrougheach having an enlarged inner counterbore portion and a smaller outerbore portion; inserting one end of each of the plurality of tubes intothe enlarged counterbore portion of a respective one of the orifices ofthe tube sheet. positioning an explosive welding charge within said oneend of each tube and within said counterbore portion of each saidorifice; and simultaneously detonating each of a plurality of saidexplosive welding charges to weld the outer surfaces of each tube at itsend to the interior tube sheet surfaces within said counterbore portion.2. The method of claim 1 wherein: the diameter of each 3. The method ofclaim 1 wherein:

the explosive welding charge comprises an annular shaped cylinder ofexplosive material positioned concentrically within the end of each tubeto extend longitudinally wholly within the enlarged counterbore portionof the respective orifice in the surrounding tube sheet.

4. A method of forming a tube sheet assembly comprising:

forming opposing tube sheets each having inner and outer surfaces with aplurality of cylindrical orifices extending between said surfaces ineach said tube sheet, each orifice having an enlarged counterboreportion extending from said inner surface and intersecting with acoaxial smaller bore portion extending from the outer surface;

inserting the ends of each of a plurality of tubes into the enlargedcounterbore portion of each of the corresponding orifices in theopposing tube sheets to extend therebetween with each tube having anannular end surface abutting an annular shoulder within its orifice atthe intersection of said portions;

positioning an annular shaped explosive charge coaxially within eachtube end and recessed from said inner surface within the surroundingcounterbore portion; and

simultaneously detonating the explosive charge within each of aplurality of tube ends to form a metallurgical bond between the outertube surfaces and the abutting tube sheet surfaces within thecorresponding ones of said orifices.

5. The method of claim 1 wherein said enlarged inner counterbore extendsinto said tube sheet from an inner surface of said tube sheet andwherein said explosive welding charge is recessed from said innersurface.

1. A method of forming a tube sheet assembly having a plurality of tubesextending therefrom comprising: forming a tube sheet with a plurality oforifices extending therethrough each having an enlarged innercounterbore portion and a smaller outer bore portion; inserting one endof each of the plurAlity of tubes into the enlarged counterbore portionof a respective one of the orifices of the tube sheet. positioning anexplosive welding charge within said one end of each tube and withinsaid counterbore portion of each said orifice; and simultaneouslydetonating each of a plurality of said explosive welding charges to weldthe outer surfaces of each tube at its end to the interior tube sheetsurfaces within said counterbore portion.
 2. The method of claim 1wherein: the diameter of each smaller outer bore portion is formed tocorrespond to the inner diameter of the end of the tube insertedtherein, and the diameter of said enlarged counterbore portion is formedto be slightly larger than the outer diameter of the end of the tubeinserted therein.
 3. The method of claim 1 wherein: the explosivewelding charge comprises an annular shaped cylinder of explosivematerial positioned concentrically within the end of each tube to extendlongitudinally wholly within the enlarged counterbore portion of therespective orifice in the surrounding tube sheet.
 4. A method of forminga tube sheet assembly comprising: forming opposing tube sheets eachhaving inner and outer surfaces with a plurality of cylindrical orificesextending between said surfaces in each said tube sheet, each orificehaving an enlarged counterbore portion extending from said inner surfaceand intersecting with a coaxial smaller bore portion extending from theouter surface; inserting the ends of each of a plurality of tubes intothe enlarged counterbore portion of each of the corresponding orificesin the opposing tube sheets to extend therebetween with each tube havingan annular end surface abutting an annular shoulder within its orificeat the intersection of said portions; positioning an annular shapedexplosive charge coaxially within each tube end and recessed from saidinner surface within the surrounding counterbore portion; andsimultaneously detonating the explosive charge within each of aplurality of tube ends to form a metallurgical bond between the outertube surfaces and the abutting tube sheet surfaces within thecorresponding ones of said orifices.
 5. The method of claim 1 whereinsaid enlarged inner counterbore extends into said tube sheet from aninner surface of said tube sheet and wherein said explosive weldingcharge is recessed from said inner surface.